There are various types of known mattress apparatuses. Of these, a water mattress apparatus is known. The water mattress apparatus has a water bag containing a liquid therein and, on load, the liquid contained in the water bag is displaced along the uneven contour of the body of a user on the water mattress apparatus in a supine position. Since, in this case, there occurs no reaction force which may be produced from the strength corresponding to that amount of compression, such as that of a mattress apparatus composed of a spring unit, no great compression force acts upon any heavy body portion of the user, such as the buttocks.
Unless the user is forced to have his or her body partially and strongly compressed, such a mattress apparatus can be used for, for example, an ill or elderly person confined to bed over a long period of time, thus preventing any bed sores upon him or her. In recent years, the water mattress apparatus has been extensively utilized in view of its advantage to which attention is paid.
The water mattress apparatus described above, usually, has a case with a water bag held in its receiving section. The case with the water bag so held is covered with an outer cover unit comprised of a cloth sewed into a bag-like unit.
The liquid contained in the water bag is non-compressive in nature during moving in a deformed water bag on application of load. For this reason, the water mattress apparatus with the water bag held in the case imparts no cushioning action resulting from a varying elastic compression to the user on load, thus failing to give any comfortable cushioning action to the user as is obtained from the spring mattress.
The liquid contained in the water bag has its temperature lowered considerably in the winter season. To this end, a heater is arranged in the case for holding the water bag and the liquid in the water bag is warmed by the heater to a temperature at which the mattress apparatus is comfortable to sleep on.
It is said that about 33.degree..+-.1.degree. C. is usually a temperature at which the user can comfortably sleep on the mattress apparatus. It is also said that the user can readily fall asleep at a temperature about 3.degree. C. higher at his or her feet than at the remaining portions of the body. There are sometimes the case where, since, in particular, a person unduly sensitive to cold is usually about 3.degree. C. lower at his or her feet than at the remaining body portions, he or she cannot enjoy a comfortable sleep unless the feet are more warmed than the rest of his or her body.
The water bag is formed of an elastic, water-proof sheet-like material, such as rubber or vinyl. When the water bag is pierced by something sharp, abruptly impacted or damaged due to degeneration over prolonged use, etc., the liquid sometimes leaks from the water bag into the receiving section of the case.
In order to prevent the liquid which flows out of the water bag from leaking outside via the receiving section of the case, a water-proof bottom sheet is usually laid on the receiving section of the case. Simply laying such a bottom sheet on the receiving section sometimes may cause the liquid which leaks via the receiving section to be absorbed by the outer cover unit so that it oozes out of the outer cover unit.
From this view point, it has been considered to cover the top-opened side of the receiving section with a water-proof upper sheet. If, therefore, the top-opened side of the receiving section is so covered with the upper sheet, it is possible to prevent the liquid which leaks via the receiving section from being absorbed.
There is, however, a possibility that, if such an upper sheet is provided between the case and the outer cover unit, the sweat from the user on the outer cover unit in a supine position as well as a moisture content from an outer atmosphere will be accumulated over an upper area of the upper sheet.
Since the upper sheet is placed in contact with the water bag, the temperature of the upper sheet is low. For this reason, the moisture content in contact with the upper sheet is produced in the form of dew on an outer surface and a considerable amount of dew is sometimes accumulated over a prolonged period of use. In such a case, however, moisture is absorbed in the outer cover unit on the upper surface side of the upper sheet and an accumulated water content may sometime ooze out on the outer cover unit. Further, the outer cover unit tends to be readily moistened, thus giving an unpleasant feeling to the user, allowing ready multiplication of germs there and providing a cause of a bad odor.
The water bag is formed of an elastic sheet such as rubber and vinyl. If, therefore, a load is exerted on the water bag, a liquid moves in the water bag, causing it to be expanded and deformed. In order to restrict the deformation of the water bag, therefore, a plurality of band-like partition members are provided therein at given intervals. That is, the partition member is fixed at one width-direction end to an inner upper surface of the water bag and at the other end to an inner lower surface. Even if, therefore, any load is exerted on the water bag, its expansion is restricted by the partition members, thus decreasing a deformation amount to a small extent.
When an impact load is exerted on the water bag by, for example, the jumping of the user onto the mattress apparatus, then the impact is applied to the fixing area between the water bag and the partition bag, thus involving a possibility of the water bag being torn open under that impact force and of the inner liquid leaking out of the water bag.